This invention relates to electrical feedthroughs of improved design and to their method of fabrication, particularly for use with implantable medical devices.
Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed case or housing to an external point outside the case or housing. Implantable medical devices (IMDs) such as implantable pulse generators (IPGs) for cardiac pacemakers, implantable cardioverter/defibrillators (ICDs), nerve, brain, organ and muscle stimulators and implantable monitors, or the like, employ such electrical feedthroughs through their housing to make electrical connections with leads, electrodes and sensors located outside the housing.
Such feedthroughs typically include a ferrule adapted to fit within an opening in the housing, one or more conductor, and a non-conductive, low bulk permeability, insulator which supports and electrically isolates each such conductor from any other conductors passing through it and from the ferrule. Each conductor typically comprises electrical wire or pin that extends through a hole extending through the insulator. The insulator is typically formed of glass, sapphire or ceramic materials and is either glassed or brazed to the ferrule and each pin and provides a hermetic seal to prevent entry of body fluids through the feedthrough and into the housing of the IMD. The IMD housing is typically formed of a biocompatible metal, e.g., titanium, although non-conductive ceramic materials have been proposed for forming the housing. The ferrule is typically of a metal that can be welded or otherwise joined to the housing in a hermetically sealed manner. Such feedthroughs are shown in commonly assigned U.S. Pat. Nos. 4,991,582, 5,782,891, and 5,866,851 and in U.S. Pat. No. 5,470,345. It has also been proposed to use co-fired ceramic layer substrates that are provided with conductive pathways formed of traces and vias as disclosed, for example, in U.S. Pat. Nos. 4,420,652, 5,434,358, 5,782,891, 5,620,476, 5,683,435, 5,750,926, and 5,973,906.
Such single and multi-conductor feedthroughs have an internally disposed portion configured to be disposed inside the housing for connection with electrical circuitry and an externally disposed portion configured to be disposed outside the housing. Each externally disposed portion of a feedthrough pin is coupled electrically with a connector element for making connection with leads, electrodes, sensors or other components.
Many of the aforementioned IMDs include elongated electrical medical leads having one or more lead conductor connected at its proximal end to a connector. The elongated lead conductor together with conductive connector and feedthrough components within the connector effectively act as an antenna that tend to pick up stray electromagnetic interference (EMI) signals. At certain frequencies, such EMI can interfere with normal IMD operations, e.g., by being mistaken for telemetry signals and cause an IMD to change an operating mode or parameter.
This problem has been addressed in certain of the above-referenced patents by incorporating a capacitor structure upon the internally facing portion of the feedthrough ferrule coupled between each feedthrough conductor and a common ground, the ferrule, to filter out any high frequency EMI transmitted from the external lead conductor through the feedthrough conductor. The feedthrough capacitors originally were discrete capacitors but presently can take the form of chip capacitors that are mounted as shown in the above-referenced ""891, ""345, ""476, and ""906 patents and in further U.S. Pat. Nos. 5,650,759, 5,896,267 and 5,959,829, for example. Or the feedthrough capacitors can take the form of discrete discoidal capacitive filters or discoidal capacitive filter arrays as shown in commonly assigned U.S. Pat. Nos. 5,735,884, 5,759,197, 5,836,992, 5,867,361, and 5,870,272 and further U.S. Pat. Nos. 4,424,551, 5,287,076, 5,333,095, 5,905,627 and 5,999,398. The electrical poles of such discoidal capacitive filters are soldered, epoxied or otherwise adhered between a feedthrough pin and the ferrule such that the discoidal filter fills the space between the ferrule and pin or pins.
After fabrication, all such feedthroughs are subjected to helium leak testing to determine whether minute leaks have occurred through defects caused by the stresses induced in handling, fitting, and brazing or glassing the components together. Helium leak testing is also conducted after the feedthrough is attached, typically by welding, to the IMD housing to detect any defects caused by the attachment process. A high integrity hermetic seal for IMD applications is very critical to prevent the ingress of body fluid vapors into the IMD housing. Even an extremely small leak rate of body fluids through or around the insulator can, over a period of many years, build up fluids that damage sensitive internal electronic components and that can cause catastrophic failure of the IMD.
When a discoidal capacitive filter is attached across the ferrule and the pin, it becomes difficult to detect any leaks through or around the insulator due to the adhesion of the discoidal capacitive filter to the ferrule and pin and due to use of a polymeric adhesive to fill the space between the facing inner end surfaces of the discoidal capacitive filter and the annular insulator. It becomes difficult to detect any helium that passes through a crack or defect in or around the insulator because the rate of helium gas passage is diminished by the polymeric adhesives filling the space and adhering the discoidal capacitive filter to the ferrule and pin.
In low voltage capacitive filtered feedthroughs or feedthrough arrays shown in the above-referenced ""361 patent, the space between the inner end surfaces of the discoidal capacitive filters and the insulator can be left empty, because electrical arcing between the feedthrough pin and the discoidal capacitive filter outer surface (typically the ground termination) does not take place.
Such an approach is not practical for high voltage capacitive filtered feedthroughs used in ICDs which conduct high voltage defibrillation shocks, for example, where the space between the inner end surfaces of the discoidal capacitive filters and the insulator are filled with epoxy to inhibit electrical arcing.
A filtered feedthrough is provided in accordance with the present invention that does not block passage of gas in a helium leak test and enables testing of the hermeticity of the feedthrough while providing insulation of the inner surface of the filter element from the feedthrough pin and ferrule. The present invention is realized in single pin filtered feedthroughs comprising a single discrete filter element coupled between the feedthrough pin and the ferrule and in filtered feedthrough arrays or multi-polar filtered feedthroughs comprising a plurality of filter elements coupled between a respective plurality of feedthrough pins and the ferrule.
The filtered feedthrough of the present invention preferably provides a pre-formed insulative barrier or spacer located between the feedthrough insulator and the filter element rather than use of non-conductive potting compound that blocks gas passage. The pre-formed insulative barrier or spacer restrains the flow of adhesive applied into the space between the lower surface of the filter element and the spacer during attachment of the filter element to the feedthrough pin and ferrule. Thus, an air space is maintained between the insulator and the pre-formed insulative barrier or spacer that leak test gas passing through defects in the insulator or the braze between the insulator and the ferrule or the pin can enter.
The gas bypass pathway preferably extends from the air space and further comprises one of one or more air gap bypassing the filter element or one or more pin hole through the ferrule wall at one or more location remote from the weldment with the IMD housing. The pin hole allows the gas applied in a hermeticity test that passes through defects in the feedthrough insulator or its attachment to the feedthrough pin or ferrule to be passed in a continuous gas pathway extending from the air space through the ferrule to gas detection equipment.
The gas pathway bypassing the filter element comprises one or more gap extending alongside and between the filter element and the ferrule. Any of the gas applied in a hermeticity test that passes through defects in the feedthrough insulator or its attachment to the feedthrough pin or ferrule passes in a continuous gas pathway extending from the air space alongside the washer and the filter element. The insulative barrier or spacer is preferably dimensioned to ensure a gas pathway bypassing it and may constitute a pre-formed insulative washer having an edge spaced at least in part from the ferrule sufficiently to allow gas passage. Any gas applied in a hermeticity test and passing through defects in the feedthrough insulator or its attachment to the feedthrough pin or ferrule into the air space is not blocked by the washer or spacer.
The pre-formed insulative washer provides an insulation layer of the lower, inner surface of the high dielectric capacitive filter or filter array and restrains flow of adhesive into the space between the filter element and the insulator to void blocking the passage of leak test gas and enabling hermeticity leak testing within a practical elapsed time.
The filter element preferably comprises a discoidal capacitor. Filtered feedthroughs and feedthrough arrays of the present invention can be employed in both high voltage and low voltage applications, and can be greatly miniaturized. The filtered feedthrough arrays or multi-polar capacitive filter arrays of the present invention can take any form including linear arrays and two-dimensional arrays.